Positive type photoresist compositions are normally compositions comprising an alkali-soluble resin and a naphthoquinone diazido compound as a light-sensitive material. Examples of such compositions include novolak type phenol resin/naphthoquinone diazido-substituted compounds as disclosed in U.S. Pat. Nos. 3,666,473, 4,115,128, and 4,173,470. Most typical examples of such compositions include a novolak resin made of cresol formaldehyde/trihydroxybenzophenone-1,2-naphthoquinonediazidosulfonic ester, as disclosed in L. F. Thompson, Introduction to Microlithography, ACS, No 219, pp 112-121.
Novolak resin, a binder, can be dissolved in an alkaline aqueous solution without swelling. The novolak resin can also exhibit a high resistance particularly to plasma etching when an image thus produced is used as a mask for etching. Thus, novolak resin is particularly useful in this application. As a light-sensitive material, a naphthoquinone diazido compound serves as a dissolution inhibitor for reducing the alkali solubility of novolak resin. But it is peculiar in that it under-goes decomposition upon irradiation with light to produce an alkali-soluble substance which rather enhances the alkali solubility of novolak resin. Because of the great change in properties resulting from irradiation with light, a naphthoquinone diazido compound is particularly useful as a light-sensitive material for positive type photoresist.
From this standpoint, many positive type photoresists comprising novolak resin and naphthoquinone diazido light-sensitive material have heretofore been developed and put to practical use. These positive type photoresists have attained sufficient results in working lines of a width of 1.5 to 2 .mu.m.
However, integrated circuits have an increased degree of integration. There is a recent tendency to require working of ultrafine patterns formed of lines of a width of 1 .mu.m or less in the production of semiconducting substrate such as SLSI. In such applications, a photoresist having a high resolution has been required to accomplish a high accuracy of reproduction of shape of exposure mask pattern and a high sensitivity for high productivity has been desired. The above mentioned prior art positive type photoresists cannot meet these requirements.